The invention relates to a magnetron cathode for sputtering ferromagnetic targets, consisting of a basic cathode body with a magnet system having a yoke and magnet poles of opposite polarity concatenated at the periphery and disposed one inside of the other on the yoke. The magnet poles are associated with pole shoes consisting at least partially of target material having emergence surfaces similar to the configuration of the magnet poles, and behind which, in the direction of the depth of the system, leaving two circumferential air gaps, a target is disposed which is concatenated at the periphery, composed of ferromagnetic metal, and has a sputtering surface; the magnet poles and target do not overlap in their surfaces of projection onto a plane parallel to the sputtering surface, and the pole shoes and the target are connected to one another in an electrically conductive manner.
Magnetron cathodes having planar or domed sputtering surfaces are sufficiently well known. In them there is provided a spatially defined arrangement of permanent magnets and/or electromagnets in such a position relative to the sputtering surface that an annular, closed tunnel of magnetic lines of force is produced over the sputtering surface, whereby the glow discharge producing the sputtering is limited to an area in the immediate adjacency of the sputtering surface and thus the sputtering rate is increased by more than a power of ten. The term "sputtering surface" refers to the effective target surface that is exposed to the glow discharge, and from which the sputtered particles emerge; this is the front surface of the target, as a rule (German publication AS No. 24 31 832).
Such magnetron cathodes have become known in a number of variants which either are of limited applicability and/or are not entirely adequate for the applications in which they would be desirable. Thus, in the known embodiment, the pole faces of the magnet system are disposed in back of the target, so that the majority of the magnetic lines of force penetrate the target face twice. Such a method of construction, however, either is not suitable, or is suitable only in conjunction with additional measures, for targets of magnetic materials, such as those needed, for example, for the production of magnetic recording tapes.
These measures can consist, for example, in making the target very thin, so that a sufficient number of magnetic lines of force can penetrate the target. This, however, necessitates frequent replacement of the target. Another possibility consists in operating the apparatus in the magnetic saturation region of the target material, but this requires extraordinarily strong magnet system, and it has not yet been possible to gain control of the distortions of the magnetic field, which change as the target material is increasingly consumed.
Furthermore, it is possible to heat the target material at a temperature above the specific Curie point, so that the magnetic lines of force can penetrate thicker target plates. The Curie temperatures are between about 400.degree. and 1100.degree. C., depending on the material, so that considerable thermal problems are involved in this approach. It is also known to facilitate the emergence of the lines of force by means of grooves in the target face (U.S. Pat. No. 4,299,678). Division of the face of the cathode into target and pole shoes by means of air gaps, however, is not provided, so that the effect is limited, and depends on the aid of temperature elevation.
U.S. Pat. No. 4,198,283 discloses a magnetron cathode in which the target, consisting of a plurality of portions, is gripped between soft-magnetic pole shoes. This substantially satisfies the requirement that the projections of targets and pole face onto a common plane parallel to the sputtering surface do not overlap. The method of gripping precludes any air gap parallel to the said plane. Thus the use of targets of ferromagnetic materials is excluded, because in such a case the magnetic lines of force would enter the target from the pole shoes in the transverse direction, so that the formation of a magnetic tunnel, or the magnetron effect, would not occur.
In a magnetron cathode disclosed by German Federal Pat. No. 30 12 935, the magnet poles of opposite polarity lie between common planes and at the same time they have in each case an endless, elongated-round configuration which can be called concentric. The magnetic lines of force between the inner and outer poles penetrate the distance between these poles. Within this distance there is disposed a geometrically similar, i.e., elongated-round target of the material that is to be sputtered. This arrangement, however, is suitable only for the sputtering of nonferromagnetic material. German Federal Pat. No. 30 12 935 also describes the possibilities for the sputtering of ferromagnetic targets by producing a second magnetic field by additional magnet devices. This second magnetic field is configured with respect to the first magnetic field and the sputtering surface such that the tendency of the first magnetic field to penetrate the magnetically permeable sputtering surface is substantially reduced. This arrangement is very expensive because of the additional magnet system required. Furthermore, the utilization of the plate-like target material would be extremely poor, so that this known method is intended for ferromagnetic target material in rod or bar form.
The German Federal publication OS No. 33 16 640, which was not prepublished, discloses a magnetron cathode in which one magnetic pole is disposed behind the central part of the target consisting of magnetic material. This central part is surrounded, leaving an air gap, by a peripheral target part which to a certain extent has the function of pole shoes. To the extent that the peripheral target part lies spatially in front of the central target part, like magnetic poles face one another in the air gap in front of the central target part, so that a closed tunnel of magnetic lines of force overlapping the central part cannot form. The magnetic lines of force, therefore, can enter the central target from the peripheral target part only in the area of the one circumferential air gap, while the maximum sputtering effect occurs precisely in the area of the air gap in whose vicinity relatively little sputtering material is located. In fact, special measures must be taken to see that material is not sputtered from the floor of the air gap, since in the case of a noncompatible material it would contaminate the deposited layers. With such a method material can be sputtered substantially only in the immediate adjacency of the air gap, so that the degree of material utilization is very low. The locally high magnetic field concentration in the area of the only air gap, which is detrimental to a large-area ablation of the target material, is even intensified by direct coupling of the target parts to the magnet poles through highly permeable components.
German Federal patent publication OS No. 32 44 691 discloses a magnetron cathode of the kind described above, in which the permanent magnets are disposed in a plane which is either formed by the sputtering surface of the target or lies in the sputtering surface at the commencement of the first sputtering process. In a variant of this principle, the permanent magnets lie in a plane in which the back of the target is situated. In all cases, the permanent magnets are situated in a zone of very great thermal stress. Since especially the present-day high-performance magnetic materials lose their magnetic properties in a temperature range between 150.degree. and 200.degree. C., which is easily reached in magnetron cathodes, it is stated in the disclosure that the system described must be cooled very effectively. Since, in connection with the prescribed direction of magnetization, the permanent magnets must not be of less than a specified axial length of about 8 to 10 mm, but on the other hand it is essential that the magnetic substance itself or any nonferromagnetic blocks mounted laterally thereof be not sputtered, the air gaps present at this point must not exceed a gap width of about 1 to 2 mm. Keeping these air gaps small, however, can be achieved, in view of the necessary length of the permanent magnets, only if the pole shoes are provided on their inner edges with collar-like prolongations. The lines of force emerging at the collar ends are deflected on a very short path into the target material from which, following the path of least resistance, they re-enter the opposite pole of the permanent magnets, since the ferromagnetic target, the ferromagnetic supporting plate and the permanent magnets are resting directly on one another without any air gaps. The consequence is a narrowing of the "magnetic trap" so that, instead of a large-area ablation of the target material, which is desirable in itself, two pit-like erosion zones result under the edges of the collars. The arrangement of the collars makes the manufacturing process considerably more expensive, which is disadvantageous inasmuch as the pole shoes themselves participate in the sputtering process, so that as a result they have a short life.
On the one hand, magnetron cathodes are greatly favored on account of their high specific sputtering output, but on the other hand the utilization of the target materials achieved on the basis of the magnetron principle is very poor. This is because, in the area of culmination of the magnetic lines of force, the plasma produces deeply eroded pits which too soon render the target unusable. An attempt has been made to remedy this by broadening the erosion pits: German Federal Pat. No. 25 56 607 has disclosed the idea of periodically shifting the pattern of the magnetic lines of force by superimposing a second, oscillating magnetic field. For the same purpose, German Federal publication OS No. 27 07 144 has disclosed the idea of shifting the magnet system periodically parallel to the sputtering surface. Lastly, German Federal publication OS No. 30 04 546 has proposed, for the improvement of the utilization of the target material, in a rotationally symmetrical magnetron cathode, to provide in addition to the main magnet system a plurality of secondary magnet systems by which the outwardly diminishing density of the lines of force of the main magnet system is said to be compensated. However, all of the above-described measures can be used only with targets of nonferromagnetic materials, because in the case of ferromagnetic materials the magnetic lines of force are unable to penetrate the material of targets having economically adequate thickness.
It is therefore the aim of the invention to improve a magnetron cathode of the kind described above such that this can be used economically, i.e., with a higher specific sputtering output and with a high utilization of material, on ferromagnetic target materials.